TOPEX/Poseidon is a satellite that measures the precise shape of the ocean's surface and how this surface changes through time. This helps the scientists to calculate ocean currents, identify climate trends, and improve weather forecasting models. A recently observed climate trend is the Pacific Decadal Oscillation.

The Pacific Decadal Oscillation (PDO) is a long-term ocean fluctuation of the Pacific Ocean. The PDO waxes and wanes approximately every 20 to 30 years. From TOPEX/Poseidon data (see below) together with other oceans and atmospheres data, scientists think we have just entered the 'cool' phase. The 'cool' phase is characterised by a cool wedge of lower than normal sea-surface heights/ocean temperatures in the eastern equatorial Pacific and a warm horseshoe pattern of higher than normal sea-surface heights connecting the north, west and southern Pacific. In the 'warm' or 'positive' phase, which appears to have lasted from 1977- 1999, the west Pacific Ocean becomes cool and the wedge in the east warms.

Image at left shows a horseshoe of higher than average (warm) water in western Pacific Ocean (red and white), and lower than average (cool) blue and purple water in the eastern Pacific Ocean. This leads us to believe we have entered the cool phase of the PDO over this past year, 1999.

The PDO is an active topic of research and satellite data, such as that from TOPEX/Poseidon, helps scientists observe and understand the phenomenon. For further information see our January 2000 press release and articles in the press.

Image courtesy of Stepen Hare and Nathan Mantua, University of Washington, units are degrees Celsius

More about the Pacific Decadal Oscillation

Q. Who named the PDO and can you give me more indepth information?

A. The term PDO was coined in about 1996 by Steven Hare at the University of Washington. He, along with colleagues Nathan Mantua, Yuan Zhang, Robert Francis and Mike Wallace discovered the pattern as part of work on fish population fluctuations. They have online papers that provide excellent information.

Q. How does this affect climate?

A. The change in location of the cold and warm water masses alters the path of the jet stream. Put simply, the jet stream in the northern hemisphere delivers storms across the United States. The PDO phase that we appear to have entered will act to steer the jet stream further north over the Western United States.

A. These will still continue, they are a pattern that can be thought of as lying on top of the large scale temperature distribution determined by the Pacific Decadal Oscillation.

Q. What is the connection between ocean height as observed by TOPEX/Poseidon and ocean temperature?

A. When the surface layer of the ocean is warmed it expands and hence results in a higher surface.

Q. Will we have a drought in southern California?

A. If the Pacific Decadal Oscillation has switched we are likely to have 20-30 years with lower rainfall that we have had since the late '70's. We will still have winter rains, but the number of really wet years is likely to decrease.

Q. I have heard that some scientists do not agree that the Pacific Decadal Oscillation has switched, what does this mean?

A. Some scientists say it's too soon to tell whether the temperature shift is part of a long-term cycle. Scientists will be studying many types of data and watching to see how temperature patterns across the Pacific ocean evolve over time.